This invention relates to a hammer of the valveless pneumatic type which is generally used for "down-the-hole" drilling.
These hammers normally comprise a hollow casing with an operatively upper and a lower end, and, which has a piston therein which reciprocates between an upper and a lower pressure chamber, and also has a bit assembly at the lower casing end and a backhead assembly at the upper casing end.
The design of such hammers usually provides for additional upper chamber space above the piston in order to prevent the pressure from rising too high when the volume decreases owing to the piston's upward movement. This space increases the volume of the upper chamber and in the conventional design results in an increase in the overall hammer length.
The outside diameter of typical hammers is usually restricted owing to the size hole that they are designed to drill, and thus there is difficulty in fitting the largest possible diameter piston inside the bore of the hammer, which is desirable in order to obtain more effective piston impact on the bit assembly. The largest diameter piston possible which could fit in the hammer, is of course a piston having a diameter marginally smaller than the inside diameter of the threaded ends of the casing. It is usually not possible to fit that piston of this size however, as a shoulder has to be provided to locate the chamber divider on the inner end thereof in the casing. This same restriction applies to the bit assembly, as a guide is usually located at the other end of the casing to secure the bit assembly.
It is an object of this invention to provide a valveless pneumatic hammer which has features which alleviate the above-mentioned problems.